The invention relates to a liquid-crystalline medium and to twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN here covers any relatively highly twisted display element having a twist angle with a value of between 160xc2x0 and 360xc2x0, such as, for example, the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021, OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays are distinguished compared with standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, associated therewith, better contrast values, and by significantly lower angle dependence of the contrast.
Of particular interest are TN and STN displays having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimized using mostly monotropic additives having relatively high vapour pressure. However, the response times achieved were not adequate for every application.
In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K33/K11 and relatively small values for xcex94xcex5/xcex5xe2x8axa5, where xcex94xcex5 is the dielectric anisotropy and xcex5xe2x8axa5 is the dielectric constant perpendicular to the longitudinal molecular axis.
In addition to optimization of the contrast and response times, further important requirements are made of mixtures of this type:
1. broad d/p window
2. high long-term chemical stability
3. high electrical resistance
4. low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex STN displays (with a multiplex rate in the region of about {fraction (1/400)}), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about {fraction (1/64)} and {fraction (1/16)} respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.
Thus, there continues to be a great demand for TN and STN displays, in particular for medium- and low-multiplex STN displays, having very short response times at the same time as a large working-temperature range, high line-line steepness, good angle dependence of the contrast and low threshold voltage which meet the above-mentioned requirements.
The invention has an object of providing liquid-crystalline media, in particular for TN and STN displays, which do not have the above-mentioned disadvantages or only do so to a lesser extent and at the same time have short response times, in particular at low temperatures, and very good steepnesses.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that this object can be achieved if use is made of liquid-crystal mixtures which comprise one or more compounds of the formula A 
and at least one compound of the formula B 
in which
Ra and Rb are each, independently of one another, H or an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted to perhalo-substituted by halogen, where one or more CH2 groups in these radicals may also each, independently of one another, be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x83xe2x80x94CHxe2x95x90CH, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another, 
Z1 and Z2 are each, independently of one another, xe2x80x94CH2CH2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94(CH2)3xe2x80x94 or a single bond,
L1 to L8 are each, independently of one another, H or F, and
Y is F, Cl, SF5, NCS, SCN, OCN, or a monohalogenated or polyhalogenated alkyl, alkoxy, alkenyl or alkenyloxy radical, each having 1 to 5 carbon atoms.
The use of the compounds of the formulae A and B in the mixtures for TN and STN displays according to the invention results in
high steepness of the electro-optical characteristic line,
low temperature dependence of the threshold voltage,
very fast response times, in particular at low temperatures.
The compounds of the formulae A and B significantly shorten, in particular, the response times of TN and STN mixtures while simultaneously increasing the steepness and reducing the temperature dependence of the threshold voltage.
The mixtures according to the invention are furthermore distinguished by the following advantages:
they have low viscosity,
they have a low threshold voltage and operating voltage, and
they effect long shelf lives in the display at low temperatures.
The invention furthermore relates to a liquid-crystal display having
two outer plates, which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5xc2x0 and 600xc2x0,
a nematic liquid-crystal mixture consisting of
a) 15-75% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 25-85% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between xe2x88x921.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below xe2x88x921.5, and
d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,
characterized in that component A comprises at least one compound of the formula A 
and at least one compound of the formula B 
in which Ra, Rb, Z1, Z2, L1, L2, L3, L4, L5, L6, L7, L8 and Y are as defined above.
The invention also relates to TN and STN displays, in particular medium- and low-multiplexed STN displays, containing the liquid-crystal mixture according to the invention.
Formula A covers, for example, compounds of the sub-formulae A-1 to A-12 
in which Ra is as defined above, and (F) means H or F.
Particular preference is given to mixtures according to the invention which comprise at least one compound of the formula A-2 and/or A-8.
In the formulae A and A-1 to A-12, Ra is particularly preferably straight-chain alkyl or alkoxy, or 1E-alkenyl or 3E-alkenyl having from 2 to 7 carbon atoms.
Y is preferably F, Cl, CN, CF3, C2F5, C3F7, CF2H, OCF3, OCF2H, OCFHCF3, OCFHCFH2, OCFHCF2H, OCF2CH3, OCF2CFH2, OCF2CF2H, OCF2CF2CF2H, OCF2CF2CFH2, OCFHCF2CF3, OCFHCF2CF2H, OCFHCFHCF3, OCH2CF2CF3, OCF2CF2CF3, OCF2CFHCFH2, OCF2CH2CF2H, OCFHCF2CFH2, OCFHCFHCF2H, OCFHCH2CF3, OCH2CFHCF3, OCH2CF2CF2H, OCF2CFHCH3, OCF2CH2CFH2, OCFHCF2CH3, OCFHCFHCFH2, OCFHCH2CF3, OCH2CF2CFH2, OCH2CFHCF2H, OCF2CH2CH3, OCFHCFHCH3, OCFHCH2CFH2, OCH2CF2CH3, OCH2CFHCFH2, OCH2CH2CF2H, OCHCH2CH3, OCH2CFHCH3, OCH2CH2CF2H, OCCIFCF3, OCCIFCCIF2, OCCIFCFH2, OCFHCCl2F, OCClFCF2H, OCClFCClF2, OCF2CClH2, OCF2CCl2H, OCF2CCl2F, OCF2CClFH, OCF2CClF2, OCF2CF2CClF2, OCF2CF2CCl2F, OCClFCF2CF3, OCClFCF2CF2H, OCClFCF2CClF2, OCClFCFHCF3, OCClFCClFCF3, OCCl2CF2CF3, OCClHCF2CF3, OCClFCF2CF3, OCClFCClFCF3, OCF2CClFCFH2, OCF2CF2CCl2F, OCF2CCl2CF2H, OCF2CH2CClF2, OCClFCF2CFH2, OCFHCF2CCl2F, OCClFCFHCF2H, OCClFCClFCF2H, OCFHCFHCClF2, OCClFCH2CF3, OCFHCCl2CF3, OCCl2CFHCF3, OCH2CClFCF3, OCCl2CF2CF2H, OCH2CF2CClF2, OCF2CClFCH3, OCF2CFHCCl2H, OCF2CCl2CFH2, OCF2CH2CCl2F, OCClFCF2CH3, OCFHCF2CCl2H, OCClFCClFH2, OCFHCFHCCl2F, OCClFCH2CF3, OCFHCCl2CF3, OCCl2CF2CFH2, OCH2CF2CCl2F, OCCl2CFHCF2H, OCClHCClFCF2H, OCF2CClHCClH2, OCF2CH2CCl2H, OCClFCFHCH3, OCF2CClFCCl2 H, OCClFCH2CFH2, OCFHCCl2CFH2, OCCl2CF2CH3, OCH2CF2CClH2, OCCl2CFHCFH2, OCH2CClFCFCl2, OCH2CH2CF2H, OCClHCClHCF2H, OCH2CCl2CF2H, OCClFCH2CH3, OCFHCH2CCl2H, OCClHCFHCClH2, OCH2CFHCCl2H, OCCl2CH2CF2H, OCH2CCl2CF2H, CHxe2x95x90CF2, CFxe2x95x90CF2, OCHxe2x95x90CF2, OCFxe2x95x90CF2, CHxe2x95x90CHF, OCHxe2x95x90CHF, CFxe2x95x90CHF, OCFxe2x95x90CHF, in particular F, Cl, CN, CF3, CF2H, C2F5, C3F7, OCF3, OCF2H, OCFHCF3, OCFHCFH2, OCFHCF2H, OCF2CH3, OCF2CFH2, OCF2CF2H, OCF2CF2CF2H, OCF2CF2CFH2, OCFHCF2CF3, OCFHCF2CF2H, OCF2CF2CF3, OCF2CHFCF3, OCClFCF2CF3.
Formula B covers, for example, compounds of the sub-formulae B-1 to B-6, 
in which Rb is as defined above.
Particular preference is given to compounds in which Rb is a straight-chain alkyl radical having 1-7 carbon atoms or an alkenyl radical having 2-7 carbon atoms. Particular preference is given to compounds of the formulae B-1, B-2 and B-4.
The use of compounds of the formulae A and B in the liquid-crystal mixtures according to the invention results in particularly low rotational viscosity values and in TN and STN displays having high steepness and fast response times, in particular at low temperatures.
Besides the compounds of the formulae A and B, component A, or the liquid-crystalline mixture according to the invention, additionally preferably comprises one or more 3,4,5-trifluorophenyl compounds selected from the group consisting of the compounds of the formulae IIa to IIk 
in which
R2 is H, an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted to perhalo-substituted by halogen, where one or more CH2 groups in these radicals may also, in each case independently of one another, be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x83xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another.
Besides the compounds of the formulae A and B, the medium according to the invention may also additionally comprise one or more compounds containing polar end groups, of the formulae II*a to II*s 
in which R2 is as defined above, and L3 and L4 are each, independently of one another, H or F. R2 in these compounds is particularly preferably alkyl, alkenyl or alkoxy having up to 7 carbon atoms.
The medium according to the invention or component A particularly preferably comprises compounds of the formulae IIa, IIb, IIc, IId, IIe, IIf, IIg, IIj, II*b, II*c, II*d, II*f and/or II*i, in particular compounds of the formulae IIa, IIb, IId, IIi, II*a and II*i.
The mixtures according to the invention may also preferably comprise one or more cyano compounds of the formulae IIIa to IIIi: 
in which R3 is as defined for Ra, and L1, L2 and L5 are each, independently of one another, H or F. R3 in these compounds is particularly preferably alkyl, alkenyl or alkoxy having 1 to 7 carbon atoms.
Particular preference is given to mixtures which comprise one or more compounds of the formulae IIIb, IIIc and IIIf, in particular those in which L1 and/or L2 is F.
Preference is furthermore given to mixtures which comprise one or more compounds of the formula IIIf and/or IIIg in which L2 is H and L1 is H or F, in particular F.
The individual compounds of the formulae A, B and of the formulae IIa-IIk, II*a-II*s and IIIa to IIIj and their sub-formulae or also other compounds which can be used in the mixtures or TN and STN displays according to the invention are either known or can be prepared analogously to the known compounds.
The compounds of the formula A have low viscosities, in particular low rotational viscosities, and low values for the ratio of the elastic constants K33/K11, and therefore result in short response times in the displays according to the invention, while the presence of compounds of the formula B of high dielectric anisotropy, in particular in increased concentrations, causes a reduction in the viscosity.
Preferred liquid-crystal mixtures comprise one or more compounds of component A, preferably in a proportion of from 15% to 75%, particularly preferably from 20% to 65%. These compounds preferably have a dielectric anisotropy of xcex94xcex5xe2x89xa7+3, in particular of xcex94xcex5xe2x89xa7+8, particularly preferably of xcex94xcex5xe2x89xa7+12.
Further preferred mixtures comprise
one or more, in particular from two to four, compounds of the formula A,
one, two or three compounds of the formula A,
one or more, in particular one or two, compounds of the formula B,
one or more, in particular from two to five, compounds of the formulae IIIa, IIIb, IIIc and/or IIIf.
Preferred liquid-crystal mixtures comprise one or more compounds of component B, preferably from 25 to 85%. The compounds from group B are distinguished, in particular, by their low values for the rotational viscosity xcex31.
Component B preferably comprises one or more compounds of the formula IV 
in which
m is 0 or 1,
R4 is an alkenyl group having from 2 to 7 carbon atoms,
R5 is as defined for Ra or, if m=1, is alternatively F, Cl, CF3 or OCF3,
L1 and L2 are each, independently of one another, H or F.
Particularly preferred compounds of the formula IV are those in which R4 is alkenyl having from 2 to 7 carbon atoms, in particular those of the following formulae: 
in which R3a and R4a are each, independently of one another, H, CH3, C2H5 or n-C3H7 and alkyl is an alkyl group having from 1 to 7 carbon atoms.
Particular preference is given to TN and STN displays according to the invention in which the liquid-crystal mixture comprises at least one compound of the formula IV-1 and/or IV-3 in which R3a and R4a each have the same meaning, and displays in which the liquid-crystal mixture comprises at least one compound of the formula IV5.
In a further preferred embodiment, the mixtures according to the invention comprise one or more compounds of the formula IV6.
Component B preferably furthermore comprises compounds selected from the group consisting of the bicyclic compounds of the formulae V-1 to V-9 
and/or one or more compounds selected from the group consisting of the tricyclic compounds of the formulae V-10 to V-27 
and/or one or more compounds selected from the group consisting of the tetracyclic compounds of the formulae V-28 to V-34 
in which R6 and R7 are as defined for Ra in the formula A, and L is H or F.
Particular preference is given to compounds of the formulae V-25 to V-34 in which R6 is alkyl and R7 is alkyl or alkoxy, in particular alkoxy, each having from 1 to 7 carbon atoms. Preference is furthermore given to compounds of the formulae V-25, V-28 and V-34 in which L is F.
R6 and R7 in the compounds of the formulae V-1 to V-34 are particularly preferably straight-chain alkyl or alkoxy having from 1 to 12 carbon atoms.
Particular preference is given to mixtures according to the invention which comprise one or more compounds of the formula B-3a and/or B-5a 
The mixtures preferably comprise 2-25% by weight, in particular 2-15% by weight, of compounds of the formula B-5a.
If desired, the liquid-crystalline mixtures comprise an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is greater than 0.2. A multiplicity of chiral dopants, some of which are commercially available, is available to the person skilled in the art for the component, such as, for example, cholesteryl nonanoate, S-811, S-1011 and S-2011 from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). The choice of dopants is not crucial per se.
The proportion of the compounds of component C is preferably from 0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to 3%.
The mixtures according to the invention may also, if desired, comprise up to 20% of one or more compounds having a dielectric anisotropy of less than xe2x88x922 (component D).
If the mixtures comprise compounds of component D, these are preferably one or more compounds containing the structural unit 2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A 38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particular preference is given to tolans containing this structural unit, as described in International Patent Application PCT/DE 88/00133.
Further known compounds of component D are, for example, derivatives of the 2,3-dicyanohydroquinones or cyclohexane derivatives containing the structural unit 
as described in DE-A 32 31 707 and DE-A3407013.
The liquid-crystal displays according to the invention preferably contain no compounds of component D.
The term xe2x80x9calkenylxe2x80x9d in the definition of Ra, Rb, R2, R3, R4, R5, R6 and R7 covers straight-chain and branched alkenyl groups, in particular the straight-chain groups. Particularly preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl, C4-C7-3E-alkenyl and C5-C7-4-alkenyl.